Pressure vessels are commonly used for containing a variety of fluids under pressure, such as storing hydrogen, oxygen, natural gas, nitrogen, propane, methane, and other fuels, for example. Suitable container shell materials include laminated layers of wound fiberglass filaments or other synthetic filaments bonded together by a thermal-setting or thermoplastic resin. A polymeric or other non-metallic resilient liner or bladder often is disposed within the composite shell to seal the vessel and prevent internal fluids from contacting the composite material. The composite construction of the vessels provides numerous advantages such as lightness in weight and resistance to corrosion, fatigue and catastrophic failure. These attributes are due at least in part to the high specific strengths of the reinforcing fibers or filaments that are typically oriented in the direction of the principal forces in the construction of the pressure vessels.
FIG. 1 illustrates an elongated pressure vessel 10, such as that disclosed in U.S. Pat. No. 5,476,189, entitled “Pressure vessel with damage mitigating system,” which is hereby incorporated by reference. Vessel 10 has a main body section 12 with end sections 14. A boss 16, typically constructed of aluminum, is provided at one or both ends of the vessel 10 to provide a port for communicating with the interior of the vessel 10. As shown in FIG. 2, vessel 10 is formed with an inner polymer liner 20 covered by an outer composite shell 18. In this case, “composite” means a fiber reinforced resin matrix material, such as a filament wound or laminated structure. The composite shell 18 resolves structural loads on the vessel 10.
FIG. 2 illustrates a partial cross-sectional view, taken along line 2-2 of FIG. 1, of a typical end section 14 including boss 16, such as that disclosed in U.S. Pat. No. 5,429,845, entitled “Boss for a filament wound pressure vessel,” which is hereby incorporated by reference. The boss 16 typically has a neck 21 and an annular flange 22. Typically, shell 18 meets neck 21 at interface 19, and flange 22 is sandwiched between the liner 20 and the shell 18. This construction secures the boss 16 to the vessel 10 and provides a seal at the interfaces between the boss 16, shell 18 and liner 20. An exterior annular groove 30 in the exterior surface 62 of flange 22 accepts a complementary exterior annular tab 32 formed in liner 20. Similarly, an interior annular groove 34 in the interior surface 60 of flange 22 accepts an interior annular tab 38 formed liner 20. The grooves 30, 34 and tabs 32, 38 secure the liner 20 to the boss 16.
This type of interlocking liner and boss structure has proved effective in certain applications, such as for compressed natural gas (CNG) fuel containers. However, in high pressure (e.g., 700 bar) service, distortions of the plastic liner material adjacent the boss has been noted, leading to some tendency of the plastic liner 20 to be pulled out of the keyway (i.e., tabs 32, 38 are pulled out of the interlocking grooves 30, 34). The distortion of this area in high pressure applications results from the presence of high pressure gas in the keyway between the liner 20 and boss 16. High pressure gas saturates the liner material and then outgases when the pressure drops. Thus, the gas permeating the interface between the liner 20 and boss 16 can then have a higher pressure than the gas within vessel 10, such as, for example, when gas is being vented from the vessel 10. As a result, the excess pressure between the liner 20 and boss 16 can cause the liner material to be forced out of the keyway. Moreover, the seal between flange 22 and liner 20 relies at least in part upon the tension of the liner 20 as wrapped around the flange 22. When the liner 20 stretches, it can cause a loss of tension and thus, a leak at the interface between the boss 16 and the liner 20.